Just recently bought a 1985 fortravel motorcoach and we can't figure out what type/how big of batteries we need. We have already tried using 2 29dc marine batteries from our boat but they quickly were zapped. Any info would be helpful.

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Not much info, but, sounds like your converter is not charging them as it should if you have it plugged in to 110 power! First, charge the batterys and load test them before you hook them up. Then, if ok, then hook them up and you need to check the voltage at the batterys after hooking them up, before you plug it in. See if you have a big draw on them, then plug it in and see what the voltage is? You should have an increase in voltage after plugging it in! Check all of the grounds, which is a job in itself, but on a coach that old, if they have not been cleaned for awhile, that alone will make a big difference! There will be grounds to the frame from the motor and from the battery bank, as you look you will see more of them, and when you see one, take the time to loosen it, brush it good with a wire brush, and tighten it back up. The grounds are very important, and clean one's will make everything work better! Hope this helps? If you do not have the means to check the system, find a good Mobile Mech. and have him check it out for you. Watch him, and see what he does, have him explane it to you, so you can do it the next time! Rail!
PS, Welcome to irv2, someone with better info will be here to help you I'm sure!!

What do you define as 'quickly'?
What were you running? Just lighting? Large inverter running a CRT TV, microwave, other appliance/energy hog?

I agree with fully charging the batteries and then having them load tested. I'd also disconnect them and check individual voltages.
Most auto parts stores are able to do that without removing the batteries, and usually will do it for free. It's probably not worth spending ~$50 on a 500amp carbon pile load tester for a once in a blue moon test.